1. Field
This invention relates to crystal oscillator circuits, and more specifically, to differential crystal oscillators.
2. Background
A crystal oscillator (XO) circuit is a critical component of an RF system and is used for reference frequency generation in transceivers. For these transceivers, responses to a frequency different from that to which the transceivers are tuned (spurious responses) are one of the challenging issues. The XO harmonics can easily couple to other RF circuits through multiple paths and appear at receiver and transmitter outputs. Compared to a single-ended structure, a differential XO circuit is desirable due to better immunities to interference and spurious responses. However, the lack of a low-impedance DC path in the XO circuit causes the circuit to latch up rather than oscillate. Conventional differential XO circuit designs have used a negative conductance (−gm) of a modified cross-coupled pair of transistors as an active device and have added high-pass filtering to avoid latching at low frequencies.